1. Field of the Invention
The present invention relates to cavity resonators, such as used in magnetic resonance imaging equipment, in which radio frequency (RF) magnetic fields are generated and received for interaction with a sample being investigated.
2. Description of the Related Art
This high frequency fields of nuclear magnetic resonance (NMR) devices have conventionally been produced by a Helmholtz pair of coils formed in the shape of a saddle. It is a well known principle of electromagnetic theory that uniform magnetic fields are produced by cylindrical current sheets whose current varies sinusoidally with the azimuth. See, e.g., Beth, "Elliptical and Circular Current Sheets to Produce a Prescribed Internal Field." IEEE Transactions of Nuclear Science, June 1967, p. 387. Because the saddle shape is only a coarse approximation of a cylindrical current sheet, the magnetic field uniformity produced by a saddle coil configuration is good only near the center of the imaging volume. Moreover, the required length of such coils causes phase shifts of the radio frequency (RF) magnetic field to occur, which further degrades the quality of the RF magnetic field. Thus, NMR devices with saddle coils cannot be used at the higher frequencies of approximately 85 MHz required for high-speed imaging methods, such as disclosed in the co-pending application, Ser. No. 937,529, filed Dec. 3, 1986 entitled "Method of High-Speed Magnetic Resonance Imaging", and U.S. Pat. No. 4,628,264, issued Dec. 9, 1986, entitled "NMR Gradient Field Modulation", both assigned to the present assignee.
U.S. Pat. No. 4,506,224 to Krause, issued Mar. 19, 1985, discloses a conductor system for generating a high-frequency magnetic field formed of an envelope surrounding at least one conductor pair. The envelope is designed to be substantially impervious to high frequencies (to eliminate undesired coupling with external components), but permeable to low frequencies (to allow the magnetic field gradient in). The conductor system geometry approximates that of a saddle coil. Advantageously, the reduced length of the conductor sections allow operation at higher frequencies such as 20 MHz. However, like the conventional saddle coil, the flow of current through the conductors forms only a coarse approximation of a cylindrical current sheet. Thus, the magnetic field uniformity is far from optimal.
U.S. Pat. No. 4,439,733 to Hinshaw et al. is an attempt to approximate the ideal cylindrical current sheet. In the disclosed structure, a plurality of conductive elements are helically wound around a frame in cylindrical relation with respect to the axis along which the static magnetic field of the system is directed. As shown in FIG. 2 of Hinshaw et al., the alternating radio frequency current flows through the conductors in the upper half of the cylinder in an opposite direction to the current flowing through the conductors in the lower half of the cylinder. The conductors are interconnected by sections of transmission line (insulated conductors) such that a standing wave, at a particular frequency, occurs across the conductive system, establishing relative amplitudes of the current in the conductive elements. This results in a substantially uniform magnetic field traverse of the axis of the magnetic resonance system.
The system of Hinshaw et al., however, is limited to low frequencies by the need to wind a many-turn toroid from a single wavelength of cable. Although higher frequencies can be achieved by exposing the center conductor at only two locations, such a coil has a low RF field homogeneity, similar to that of a saddle coil. See, P. Roeschman, Third Annual Meeting of the Society of Magnetic Resonance in Medicine, New York, N.Y., Aug. 17-23, 1984, p. 634.
A further prior art design, which is also an approximation of an ideal cylindrical current sheet, is the so-called "birdcage" resonator disclosed in an article by C. Hayes, entitled "An Efficient, Highly Homogeneous Radiofrequency Coil for Whole-Body NMR Imaging at 1.5T", J. Magn. Reson., Vol. 63, pp. 622-28 (1985). This design can be considered to be a further development of the Aldeman-Grant coil. J. Magn. Reson., Vol. 36, p. 447 (1979). The "birdcage" consists of two circular end rings connected by a number of equally spaced straight segments, each which includes a capacitance and has an inherent inductance. An inductance is also present in each of the individual segments of the end rings. The resonance of such a structure is analyzed by Hayes et al. by considering wave propagation around the cylinder. Each of the end ring segments introduces a phase shift. If the total phase shift around the end rings is an integer multiple of 2.pi., a standing wave is created, which generates currents in the straight segments proportional to sin .theta., thus approximating the ideal current sheet discussed above. Unfortunately, the end rings of the birdcage resonator disclosed by Hayes carry currents which do not approximate the ideal current sheet.